Abstract: To solve the differ problem of the vibration isolation effect of the resonant section and high frequency section in traditional three parameter vibration isolator, a model of variable rod length X-shaped structure vibration isolator with geometric nonlinear characteristics was proposed. Firstly, a dynamic response analytical model was established based on the harmonic balance method. And then, comparing with the time-domain numerical solution and the simulation data got from the multi-body dynamics software ADAMS, the analytical solution was proved accurately. On the basis, the equivalent stiffness coefficient, equivalent damping coefficient, force transfer rate, dynamic force transmitted to the foundation under multi frequency steady-state excitation and the influence of key design parameters on the transmission characteristics of the vibration isolation system were analyzed finally. The results show that the equivalent stiffness and equivalent damping of the vibration isolation system with variable rod length X-shaped structure can change monotonically. The resonant peak value of variable rod length X-shaped structure vibration isolation system decreases significantly, the frequency does not change, and the corresponding time-domain stress response also presents the same variation characteristics.